Some previous frequency measurement apparatus have utilized a single sampler. See, for example, U.S. Pat. No. 3,984,770 issued on Oct. 5, 1976 to David C. Chu and entitled "Frequency Measurement Using Phase Continuous Frequencies Switching". These single sampler systems are much to be desired over the prior art dual sampler systems such as that shown in U.S. Pat. No. 3,836,758 issued on Sept. 17, 1974 to Richard F. Schneider and Arthur R. Bloedorn and entitled "Wide Frequency Range Counter System Utilizing Automatic Range Searching and Loop Compensation". This is because significant cost savings can be achieved by eliminating one of the samplers required.
However, previous single sampler systems have typically utilized techniques wherein the single sampler is alternately driven by signals of two different preselected frequencies. The disadvantage of this technique is that changes in the input signal frequency due to frequency modulation or drift between the time the input signal is sampled using the signal of the first frequency and the time that the input signal is sampled using the signal of the second frequency may result in errors in the calculation of the harmonic number and sideband of the input frequency.
In accordance with the preferred embodiment of the present invention, a signal of unknown frequency is input into a sampler. The sampler is driven by a sampling signal which is caused to vary in an irregular or random manner by a control signal. The IF output of the sampler is gated to a counter using synchronous detection by the control signal. The correlation of the modulation applied to the sampling signal with the resultant frequency modulation of the IF signal cancels out frequency modulation of the input signal which may be present. Using the IF frequency value from the counter, one can compute N and the sidebands, since the IF frequency will vary in the same way as the sampling frequency, except for a scale factor which is equal to the harmonic number. If the frequency of the input signal is also varying, the effect of this variation on the measurement accuracy will be greatly diminished by the correlation process .